The capability of detecting and continuously measuring low levels of chloride, sulfate, phosphate and nitrate has been needed for water treatment control in the power industry for many years. The available titration or colorimetric procedures are not continuous measurements and have not possessed the necessary sensitivity or accuracy. So-called cation conductivity procedures have been used in the past as an indication of condensate leakage (inorganic comtamination) but, as with many such procedures, the errors involved with their use have not been fully recognized nor have corrective measures been applied to obtain precise results.
The use of once-through and supercritical boilers, pressurized water reactor (PWR) nuclear and cogeneration systems has brought to light problems of corrosion and deposits on surfaces of steam boilers and generators as a result of which stress-corrosion cracking of turbine blades and circulation troubles in the system occur, as in the denting (cracking) of Iconnel 600 in PWR heat exchangers. Chloride, sulfate, phosphate and nitrate in the high temperature feedwater, normally found in water, are responsible for this corrosion, for the harmful deposits, and for the deterioration of auxiliary equipment. Resulting mechanical failure, such as boiler tube eruption, may cause injury to operating personnel and serious economic disruption for the public.
The foregoing problem of stress corrosion cracking of turbine blades in large steam turbines has revealed an urgent need for reexamination of the allowable upper limits of steam purity. Heretofore, a maximum of 10-30 .mu.g/l (micrograms per liter) sodium (as Na.sup.+), equivalent to 15-45 .mu.g/l chloride (as C1), was considered allowable. Recently, turbine manufacturers have specified chloride ions plus sulfate ions (C1.sup.- +SO.sub.4.sup.=) limits of 5 .mu.g/l or less.
Corrosion studies have recognized the importance of C1.sup.- and SO.sub.4.sup.= ions in promoting the corrosion of metals in water. The measurement of the Na.sup.- (sodium) ion may not provide a correct estimate of anion contamination, as other cations such as potassium, calcium and magnesium may be present in combination with C1.sup.- and SO.sub.4.sup.= rather than Na.sup.+ alone. Heretofore, the deleterious effect of calcium and magnesium impurities associated with chloride and sulfate has not been recognized or considered important in steam purity measurement. The recognition of such effect is important in the determination of feedwater contaminants, and is taken into account by the present invention.
So far as applicants are aware, the present invention achieves for the first time a greater sensitivity and accuracy of conductivity measurements based on chlorides, sulfates, and nitrates in feedwater than heretofore possible. The present invention is able to detect and continuously measure extremely low levels of 1 to 5 .mu.g/l (micrograms per liter) chlorides, sulfates and nitrates (equivalent to 0.6 to 3 .mu.g/l sodium). Heretofore, there has been no procedure for detecting chlorides, sulfates and nitrates at this very low level; still less, on a continuous measuring basis.
The apparatus of the present invention is designed to fill an analytical and operational need for control of contaminants and water treatment in large fossil fueled boilers, particularly once-through type or PWR (pressurized water reactor) nuclear systems. Up to the present time there has been no satisfactory method of continuous sampling and measuring the purity of saturated and superheated steam generated by these systems, due to the physical characteristics of the boiler design and the superheated steam used. Hence there is a present need to monitor the feedwater influent rather than the steam or steam condensate effluent. The present invention satisfies such need.
U.S. Pat. No. 3,158,444 granted Nov. 24, 1964 describes a method of and apparatus for determining steam purity. Although the apparatus illustrated therein bears a superficial resemblance to the apparatus of the present invention hereinafter illustrated and described it should be understood that the apparatus disclosed in the aforementioned patent is not designed for nor can it be used to test high temperature feedwater (approx. 350.degree. F. and above) in place of steam. In using the apparatus of U.S. Pat. No. 3,158,444 for the determination of steam purity, it was found that the superheated steam deposits insoluble salts, such as sodium chloride and sulfate on the sampling line to the measuring instrument and in the instrument itself. This problem is described in an article entitled "The Prevention of Errors in Steam Purity Measurement Caused by Deposition of Impurities in Sampling Lines," by R. V. Cobb and E. E. Coulter published by the American Society for Testing Materials, Philadelphia, PA., Proc. ASTM61, 1386--1395(1961). The technique for avoiding these errors described in the foregoing article is complicated and has never been used practically, so far as applicants are aware. This problem does not exist in the practice of the present invention.